This invention relates to the art of electric arc welding and more particularly to a device for monitoring the voltage sense leads of the power source during a welding operation. In addition, the invention involves the novel method of monitoring the continuity of the voltage leads using the unique monitoring device.
In the electric arc welding technology, a power source passes a current between an electrode and a work piece. Often, the electrode is a continuous welding wire drawn from a supply of welding wire, such as a drum or reel, and passed through a contact tip on its way to being melted and deposited onto the work piece. In this type of welding procedure, the power source of the welder includes a first stud connected to the electrode, usually through the contact tip, and a second stud connected to the work piece Connections are by welding cables, which cables may be quite long and include a variety of impedance variables, such as inductive reactance based upon length, position and shape of the cables. When performing a welding process, the power supply receives a current command to create a particular pulse wave between the electrode and work piece. One of the more common power sources is the Power Wave sold by The Lincoln Electric Company of Cleveland, Ohio. Such welder must accurately control the pulse shape or waveform by controlling the voltage to a pulse width modulator operated at a frequency exceeding about 20 kHz. To assure the desired welding operation constituting specific waveforms between the electrode and work piece, the command signal is created based upon a feedback from the actual welding operation. This feedback involves the arc current and/or the arc voltage. Measurement of the arc current for feedback control presents minor distortions, since the current is in a series circuit and is zero when the cables are not connected. However, the arc voltage between the electrode and work piece can not be determined by the voltage between the output studs of the power source. Voltage is affected by not only the impedance of the cables, but also the choke and other impedance creating components in the welding operation. To assure an accurate feedback of arc voltage, it is common practice to use remote voltage sensing leads directed from the controller of the power source to the electrode or contact tip and the work piece. The voltage of these leads determines the command signal to the power source from the controller. Consequently, disastrous results affecting the quality of the weld can occur if the voltage sense leads become disconnected from the welding circuit. Since the voltage command to the power source is determined by the voltage feedback signal, a broken or disconnected sense lead will provide a decreased feedback signal indicating a drop in the arc voltage. Consequently, the voltage command signal to the power source must be increased to compensate for the presumed decrease in arc voltage. This can cause the electrode to melt too rapidly for the advancing wire feed speed so the incoming wire melts into the contact tip. Such event is especially undesirable in automatic or robotics welding. The melt back of the electrode into the contact tip melts the contact tip to produce molten copper that migrates into the weld pool and results in an undesirable metallurgy for the resulting weld. Consequently, it is essential for automatic welding, such as by robot, that the operator be alerted when a voltage sense lead is disconnected or otherwise interrupted. In the past this problem has been addressed by an external device or box that is connected to the studs of the welder and receives remote voltage readings from the leads. If the voltage at the studs is greater than the remote reading plus a normal voltage drop associated with the welding cables, the power source is turned off or the operator is flagged to indicate problems with the welding operation demanding immediate attention. The most common defect is when the voltage lead to the work is broken or disconnected. This external circuitry for verifying the proper status of the voltage sense lead has disadvantages. It requires an additional component or box that is mounted onto the external portion of the power supply and requires skill by the end user for proper installation. It is expensive and time consuming to connect the box to the voltage leads and to the studs. In addition, this component must be monitored which is also expensive. Welder customers do not desire add-on components which therefore drastically reduces the desirability of the prior attempts to determine when the voltage sense leads are not properly connected.
The present invention overcomes the disadvantages of the prior art wherein an external box is needed for monitoring the voltage sense leads. In accordance with this invention, a signal representing the voltage which should appear across the leads or at the two power source studs is calculated by knowing the primary voltage, secondary current, turns ratio of output transformer and the duty cycle of the switching network of the power source. After this theoretical voltage is determined for a given time during a specific weld cycle, the synchronized theoretical voltage signal is then compared with the sensed actual voltage at either the studs or the voltage leads for this same given time. Referring specifically to the voltage leads, if one lead becomes disconnected the control circuit will see one-half the actual voltage of the welding operation. If both leads are disconnected, the control circuit will see no voltage. Consequently, by comparing the theoretical calculated voltage with the actual voltage at the voltage sense leads, the proper connection of the sense leads can be monitored on a continuous basis. If the voltage provided by monitoring the voltage sense leads is drastically under the theoretical voltage, the invention terminates operation of the welder. The invention is an executive program that is performed by the digital signal processor in a control board of the type used in most modern welders. The Power Wave welder has such control. The Power Wave is shown in Blankness U.S. Pat. No. 5,278,390, incorporated by reference herein. Thus, by merely calculating the desired theoretical voltage at a given time and reading the voltage at the sense leads at this same time, the program of the present invention sets a status bit in the control program of the welder to terminate operation of the welder immediately. To prevent burn back of the electrode into the contact tip, the total test sequence must be implemented to determined whether the status bit is to terminate operation of the welder within a time less than the time for electrode burn back into the tip. In most welding applications, burn back requires over 50 ms depending upon the type of wire, the wire diameter and the amount of original stick out. Consequently, the algorithm constituting the test sequence is completed within a time substantially less than 50 ms. In practice, this cycle time is less than 5.0 ms. In addition, the invention performs the total program for the test sequence periodically during the welding process to give a real time reading of the voltage at the leads. A total executive program is implemented in a time substantially less than the burn back time and the period of implementing the test sequence is substantially greater than the burn back time. In other words, periodically the test sequence of the present invention is performed. This implementation of the sequence involves a cycle time substantially less than the burn back time. Thus, if there is a problem with the continuity of the sense leads, the welder power source will be terminated prior to the time necessary for electrode burn back. To immediately detect a lead separation the repeat period of the lead sequence is greater than 5 ms, but less than 50 ms.
In practice, there is an exception to the general operation described above. When there is a short circuit operation, there is not enough power available to cause problems with overheating of the electrode. Thus, in accordance with a secondary aspect of the invention, the theoretical voltage on a real time basis is compared to a reference voltage, such as a voltage less than 20 volts, to determine whether the weld cycle is commanding a short circuit. If a short circuit is being required, as determined by the real time theoretical voltage, the test sequence constituting the essence of the present invention is bypassed. This is a short circuit interrupt of the program. When the welding waveform calls for a termination of a short circuit, the theoretical calculated voltage will be drastically increased so the short circuit interrupt of the program forming the present invention will terminate the test sequence of the invention.
In accordance with the invention and to accomplish the procedure set forth above, there is provided a device for interrupting operation of the power source of an electric arc welder of the type passing a current between an electrode and a work piece. The electrode is normally a welding wire passed through a contact tip on its way to the arc between the electrode and work piece. The power source includes a first stud connected to the electrode by a first cable and a second stud connected to the work piece by a second cable to perform a welding process implemented by a command signal to the power source, such as the command signal to a pulse width modulator. The welder of the present invention further has a first voltage sense lead connected to the electrode and a second voltage sense lead connected to the work piece. The primary purpose of the invention is to monitor the continuity of these two leads. They are used to develop the command signal to the power source. If they are not properly connected, the command signal will be distorted. A secondary purpose of the invention is to monitor the output studs of the power source as was done in the prior art. To accomplish the stated primary and secondary objectives, the invention includes an executive program to implement a test sequence that is performed by the digital signal processor of the controller for the welder. This novel inventive sequence involves a program routine performed in the digital signal processor of the welder to create a first signal representing a theoretical voltage value for the computer welding process being performed. This theoretical voltage will change during the welding process according to the desired current shape of the elected welding process. Consequently, the theoretical voltage is for a given time in the waveform of the welding process. For instance, when performing a pulse weld, the theoretical voltage will change during each pulse that is fashioned by the pulse width modulator. Thus, the invention is performed in real time to determine the theoretical voltage at a given instance for use in the test sequence. The invention includes a sense circuit for generating a second signal representing the voltage at the first and second sense leads for the given time. The voltage is digitized and synchronized. In the test sequence, a program routine calculates the real time mathematical relationship of the synchronized first and second signals. Thus, a real time theoretical voltage signal is calculated based upon the portion of a desired wave shape being outputted by the power source. At the same time, the voltage leads are monitored by creating a signal representing the voltage at the leads. These two real time values are compared to the other mathematical relationship, which in practice is a percentage. Then, a threshold circuit is used to set a threshold value of this mathematical relationship so that a decision circuit in the program will create a welder termination fault signal when the mathematical relationship is equal to or less than the threshold value. The test sequence is processed digitally by software commands by the computer of the digital signal processor in the welder. The word xe2x80x9ccircuitxe2x80x9d means a digital series of steps to perform the circuit objective. This test sequence is performed as a software program in the standard digital signal processor available in most modern welders. Especially welders of the type that have a pulse shape generator for controlling the pulse shape of the current through the welding operation. In accordance with an aspect of the invention, the mathematical relationship is a percentage which is approved for operation of the welder if the percentage is greater than 50 or 60%. Should the mathematical relationship be less than 50 or 60%, this is an indication that one of the sense leads is not connected. The invention then proceeds to create the fault signal terminating the operation of the welder.
In accordance with another aspect of the invention, the inventive test sequence is performed periodically during the welding process and has a cycle length of substantially less than the burn back time for the welding operation. This burn back time is normally in the range of at least 50 ms. Thus, the algorithm constituting the test sequence, in accordance with this aspect of the invention, is performed in a cycle length of time less than about 50 ms. In practice, the test sequence is implemented at a period of less than about 50 ms. The cycle length is substantially less than 5 ms and generally in the range of about 1.0-4.0 ms. Preferably, the period is 5.0-20 ms.
The invention can be explained from a different prospective. There is provided a device for controlling the operation of the power source of an electric arc welder for passing a current between an electrode and work piece from a first stud connected to the electrode by a first cable and a second stud connected to the work piece by a second cable. The welder has an output transformer with a known turns ratio, a primary winding and a secondary winding. This welder is provided with a first voltage sense lead connected to the electrode and a second voltage sense lead connected to the work piece. So far this aspect of the invention is consistent with general welding technology. The invention involves the use of an executive program implemented by the digital signal processor of the welder to implement a test sequence with a cycle time of less than 50 ms. This test sequence involves a program routine for comparing a real time theoretical voltage determined by the pulse waveform at a given time and the voltage on the sense leads at the same time. Another routine terminates the operation of the power source when the synchronized theoretical voltage substantially exceeds the sense lead voltage. There is also provided a timer to perform the executive program at a period of less than 50 ms. In accordance with the invention, the theoretical voltage is a calculation as a snapshot at a given time including the input voltage to the primary winding of a transformer, the turns ratio of the transformer and the duty cycle of the power source at that particular instance. The voltage of the leads is also sensed at this time. In accordance with an aspect of the invention, the cycle time of the test sequence is in the general range of 1.0-4.0 ms, while the period is in the range of 5-50 ms. In practice, the range of the period is 5.0-20 ms.
In accordance with a further aspect of the present invention there is provided a method for comparing a calculated snapshot or synchronized theoretical voltage for a portion of a waveform being processed by an electric arc welder and comparing in a digital signal processor the instantaneous or real time theoretical voltage with the sense voltage from the actual voltage on or between the standard voltage sense leads of the welder. The test sequence turns off the welder when the voltage from the sense leads is substantially less than it should be based upon the calculated synchronized theoretical voltage. In this method, the relationship of the instantaneous or real time theoretical voltage to the voltage read from the sense leads is a percentage. If the percentage of the read or monitored voltage is less than a set amount, such as 50-60%, of the intended theoretical voltage, then operation of the welder power source is terminated. In accordance with another aspect of the method, the instantaneous or real time theoretical voltage is compared with the set voltage, such as 18 volts to determine whether a short circuit is being outputted from the power source. If so, then the test sequence is terminated because the theoretical voltage will be quite low and does not require power source interruption and can not properly drive the test sequence.
The primary object of the present invention is the provision of a device for terminating the operation of the power source of an electric arc welder when a sense lead is disconnected. The device is a sequence implemented digitally and compares a theoretically calculated snapshot or synchronized voltage to the read voltage from the voltage leads and/or the studs. The sequence terminates the operation of the welder when the monitored voltages do not reach a certain level of the theoretical real time calculated voltage.
Another object of the present invention is the provision of a device, as defined above, which device is implemented in the digital signal processor of a modern welder. It needs not be in the form of an add-on component or control box external of the welder.
Another object of the present invention is the provision of a method of operating the device, as defined above, which method is easy to implement, consistent in operation and does not require an external component or box on the welder.
Yet another object of the present invention is the provision of a method for monitoring the voltage on voltage sense leads and comparing this voltage to a calculated theoretical voltage at a given time to terminate operation of the welder when the sensed voltage does not reach a pre selected mathematical relationship to the theoretical voltage. The theoretical voltage for a given portion of the waveform involves a calculation using the inputs for a primary voltage to the transformer of the welder, the turns ratio of the transformer and the duty cycle of the power source. In practice, a constant may be used for this calculation to produce a theoretical snapshot voltage. A constant is used to convert the theoretical voltage to the level of a voltage being monitored. This method can also be used for monitoring the voltage on the studs of the power source as a secondary implementation of the invention.